A wind turbine known in the art comprises a tapered wind turbine tower and a wind turbine nacelle positioned on top of the tower. A wind turbine rotor with a number of wind turbine blades is connected to the nacelle through a low speed shaft, which extends out of the nacelle front as illustrated on FIG. 1.
The natural critical frequency (also known as the eigenfrequency or resonance frequency) of a wind turbine tower is a known issue in the wind turbine industry.
E.g. a 50 meter tall wind turbine tower will have a tendency to swing back and forth, say, every three seconds. The frequency with which the tower oscillates back and forth is also known as the eigenfrequency of the tower. The eigenfrequency among other things depends on the height of the tower, the thickness of its walls, the type of steel, and the weight of the nacelle and rotor. Each time a rotor blade passes the wind shade of the tower, the rotor will push slightly less against the tower. If the rotor turns with a rotational speed such that a rotor blade passes the tower each time the tower is in one of its extreme positions, then the rotor blade may either dampen or amplify the oscillations of the tower.
The rotor blades themselves are also flexible, and may have a tendency to vibrate at a frequency of e.g. 1 to 2 Hz, which under certain circumstances may further amplify or dampen the tower oscillations. In worst case several factors acting simultaneously could make the tower oscillate to a degree that the tower or other parts of the wind turbine is damaged, that the life of the tower or other parts are severely reduced or at least reduce the efficiency of the wind turbine.
One way of dealing with this problem is to take different measures during the design phase of the wind turbine e.g. by making the tower stiffer, by reducing the weight of the nacelle and rotor or other. But these measures often collide with other desired qualities of the wind turbine such as low production cost, high efficiency and other.
Other way of dealing with this problem has therefore been developed and one of these is disclosed in German patent application No DE-A 10309825. This application disclose a way of damping of tower movements by three stay wires connected to individual foundations in the ground surrounding the wind turbine, each fitted with a self-controlling hydraulic damper. The dampers ensure that the movement of the light and flexible tower does not become so large and uncontrollable, that they endanger the wind turbine. But for this system to work the stay wires has to be relatively long and fixated relatively far from the tower for the dampers to see sufficient motion to function properly. This is of course disadvantageous in that the wind turbine hereby will occupy more space and in that esthetically, this design is much undesired. Furthermore, the entire system and especially the separate foundations for the stay wires will increase the overall cost of the wind turbine considerably making this design less cost-efficient, particularly with off-shore wind turbines.
An object of the invention is therefore to provide for a wind turbine without the mentioned disadvantages.
Especially it is an object of the invention to provide for an advantageous and cost-efficient technique for reducing the risk of tower oscillations causing damage to the wind turbine or reducing its efficiency.